Boiler control system



J. c. BARBER E'rAL- 3,545,207 .BOILER CONTROL SYSTEM 9` 2 snees-sheef 1`INVENTUM Jusrus c. BARBER `THERON w. JENKINSJR.

r AGENT Dec. `8, 197

Dec. `1970 J. c. BARBER ET AL4 `BOILER CONTROL SYSTEM A Filed July 23,1969 United States Patent O 3,545,207 BOILER CONTROL SYSTEM v Justus C.Barber, King of Prussia, and Theron W. Jenkins,

Jr., Ambler, Pa., assignors to Leeds & Northrup Company, Philadelphia,Pa., a corporation of Pennsylvania Filed July 23, 1969, Ser. No. 843,946

Int. Cl. F23n 1/10 U.S. Cl. 60--106 10 Claims ABSTRACT OF THE DISCLOSUREA control system responding to the energy demand of the turbine toprovide separate demand signals for each of the boiler inputs asrequired to meet the turbine demand and modifying the boiler inputs whenone of them deviates from its demand value by a predetermined allowablelimit so as to maintain all inputs in the proper relationship andmagnitude. Also, when the deviations occur, the turbine input ismodified to keep the turbine input in balance with the boiler input.

BACKGROUND OF THE INVENTION This invention relates to systems forcontrolling the boiler inputs and the turbine throttle valve inaccordance with the energy demand established for the turbine to providethe desired electrical output of the generator driven by the turbine.More specifically, this invention is concerned with the establishment ofseparate demand signals each indicative of the desired value of one ofthe inputs to the boiler and the control of the turbine throttle valveto maintain a balance between the boiler inputs and the turbine input.The level at which that balance is established is controlled inaccordance with the yenergy required from the system. The invention isparticularly concerned with the control of the boiler inputs and theturbine throttle valve under those conditions in which one or more ofthe inputs is limited, particularly as to magnitude or rate of change.

For the purposes of this description, the term energy is used to meanenergy per unit of time, more accurately referred to as power.

The boiler control systems used in the past have generally incorporatedcomplicated arrangements for maintaining the desired relationshipbetween the boiler inputs under conditions in which the magnitude of theindividual inputs or their rate of change may be limited. The presentinvention provides a simplified and more comprehensive means forcontrolling the boiler inputs and the turbine throttle valve under suchconditions.

It is an object of this invention to provide an improved boiler controlsystem.

It is another object of this invention to provide a simplied controlsystem for maintaining the required boiler inputs and the requiredthrottle valve opening for the turbine connected to the boiler so as tomaintain, under conditions of limitation as to the magnitude and/orextent of change which can be made in any one of the inputs, the properrelationship between each of the inputs as well as the properrelationship between those inputs and throttle valve opening itself.

SUMMARY OF THE INVENTION In carrying out this invention there isprovided a boiler control system which utilizes a measure of the boilerload to produce a signal representing the desired boiler demand. Thereis derived from that signal separate demand signals associated with theseparate inputs to the boiler so as to control those inputs to have apredeter mined relationship as well as to meet the boiler load whilemaintaining the desired steam pressure at the boiler output. Theimprovement comprises means for modifying the desired boiler demandsignal to produce a signal that represents a required boiler demand andto produce from that signal separate demand signals for each boilerinput. This means would include means for producing an error signalwhich varies in accordance with the difference between a signalresponsive to a measured value of a boiler input and the correspondingseparate demand signal. When that error signal exceeds a predeterminedallowed deviation, the modification of the desired boiler demand signalis such that the magnitude of the required boiler demand signal which isproduced changes the separate demand signals derived therefrom so as tomaintain the boiler inputs in their predetermined relationships.Additionally, means can be provided which are responsive to thedifference between the desired boiler demand signal and the requiredboiler demand signal and which are operable to control the steam flow tothe turbine so as to tend to maintain the difference between the desiredand required boiler demand signals within a predetermined tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a boilercontrol system.

FIG. 2 is a detailed circuit diagram of that portion of FIG. 1 whichprovides for the improvement in the operation of the control system ofFIG. l.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 the boiler 10 isprovided `with the usual inputs such as a fuel flow through line 12which is com bined at the burner 14 with the air flowing through line 16so as to provide the necessary heat input to the boiler while thefeedwater flow is provided through line 18. The air ow input throughline 16 is subject to control by the adjustment of valve 20 by thecontroller 22 which is shown as a controller providing both proportionaland integral action as noted by the notationm"`PI in block 22. The airflow through line 16 is measured by the ilowrneter 24 which by means ofthe associated pressure taps responds to the pressure differentialestablished across the flow restriction 26 inline 16.

Similarly, the fuel flow in line 12 is subject to control by theadjustment of valve 30 by the controller 32 which is similar tocontroller 22. Also, the fuel flow is measured by owmeter 34 in responseto the pressure drop across the orice plate 36, the assumption beingthat the fuel supplied is either a liquid orl a gaseous fuel.

The feedwater flow through line 18 is subject to control by the valve 40in response to the action of the controller 42 which is a controller ofthe same type as those utilized to control both the fuel and the airflow, namely controllers 22 and 32, respectively. Similarly, thefeedwater ow is measured by the owmeter 44 in response to the pressuredifferential across the orifice plate 46.

The concentration of oxygen in the fuel gases is measured at the boilerstack 48 by the oxygen concentration measuring device 50.

Still other measurements are made of the boiler outputs. For example,the temperature measuring device I52 responds to the temperaturedetected by thermocouparticular control system shown in FIG. l is ameasurement of the pressure in the iirst stage or impulse chamber of theturbine 68 which is shown as being made by the pressure measuring device70 which is connected by the tap 72 to the iirst stage or impulsechamber of turbine 68. The pressure measured by the device `62 is notedas Pt while the pressure measured by the device 70 is noted as P1.

As shown in FIG. l, the turbine 68 which is supplied with steam fromboiler by way of line 60 is mechanically coupled to the generator 74 toproduce an electrical output on the output lines 76. The power output onthe lines 76 is measured by wattmeter 80 to provide on line 81 a signalrepresentative of the actual output of the generator, namely Ga.

For the purpose of establishing the desired output of the generator 74there is provided at terminal 82 a signal Gd representing the desiredgeneration for generator 74. The signal Gd may be obtained from a simplehand-set potentiometer, for example, or as an output from a complicatedload dispatch computer.

As in most power systems, the mechanical coupling between the turbineand the generator, namely the coupling 84, rotates at a speed which isutilized as one input to a governor mechanism represented by the block86. The mechanical output of I86 by means of the mechanical coupling 88controls the position of the throttle valve 90 in the steam output line60 and the valve 90 in turn controls the steam ilow to the turbine 66.

The speed governor 86 is normally subject to adjustment of itscharacteristic in response to signals from the load dispatch systemwhich establishes the signal Gd so that the particular opening whichexists in valve 90 for a particular frequency or a particular rotationalspeed of mechanical coupling `84 may be modied so as to change theelectrical .power output in response to the changes in the signal Gdwhile maintaining a predetermined frequency in the electrical system.The modification of the governor characteristic is provided by the inputsignals on lines 91 and 92 which input signals are provided to thegovernor motor in the governor 86 so as to modify its speed responsecharacteristic.

In providing the required signals on lines 91 and 92 the signals Gd andGa are compared in the comparator 96. The difference between those twosignals then appears on line 98 as an input to controller 100 which is acontrol having proportional, integral and derivative response, asindicated by the notation PID in block 100. That controller provides anoutput on line 102. The output on line 102 is introduced as one input tocomparator 104. The other inputs to comparator 104 are the signal Gdwhich is supplied on line 106 and a signal Pl/Pt which is supplied online 108. The signal on lines 106 and 108 are compared and thedifference between them is added to the signal on line 102 so as toprovide the output of comparator 104 on line 110. That output s then theinput to the controller 112 which is shown as having both a proportionaland integral response.

The signal provided on line 108, namely Pl/Pt, is a signal which isindicative of the effective opening of the throttle valve 90 since it isderived by dividing the output of the rst stage or impulse chamberpressure Ineasuring device 70 by the output of the throttle pressuremeasuring device 62 in the dividing network shown at block 114. Theoutput of the dividing network 114 appears on line 116 which isconnected to line 108. Reference should be made to the description inthe |U.S. Pat. 3,247,671 which issued to James H. Daniels on Apr. 26,1966, for a more complete description of the signiilcance and the mannerfor producing the signal Pl/Pt on line 116'.

The controller 112 may be of the type disclosed in U.S. Pat. 3,008,072and may be designed so as to produce on its output lines 120 and 121electrical pulse signals whose duration depends upon the extent ofcontrol action called for by the controller 112. The line suppliespulses to relay operator 122 by way of relay contact 124a which isnormally subject to actuation by relay operator 124 under conditions tobe described later. When the relay operator 122 is energized, its relaycontact 122a is closed so that the potential supplied at the terminal126 can be connected through relay contact 128a and relay contact 130ato line 91 and hence to the governor motor in governor 86. The relaycontacts 128a and 130:1 are subject to being opened under conditionswhich will be described later. Any pulses which are provided fromcontroller 112 to line 91 causes the governor to act in such a way as todecrease the opening in the valve 90 for operation of the turbine at aparticular speed and hence at a particular electrical frequency on thelines 76 so as to decrease the power output of the generator 74.

In a similar Way the output from the controller 112 through line 92 togovernor `86 will cause the governor to open the valve 90 to a wideropening so as to provide for an increased output from generator 74. Thepulses provided to line 92 from the controller 112 are supplied by wayof line 121, relay contact 134a, which is subject to the actuation bythe relay actuator 134 so that the pulses supplied on line 121 when therelay contact 134a is closed causes an energization of the relayactuator 132 to close the relay contact 132a and thereby connect thepotential supplied at terminal 126 through the closed contact 132a, andthe contact 130b as well as contact 128b to line 92. In FIG. 1, thecontacts 130g and 130b are shown in the state which is normal when theiractuator 130 is de-energized. Also, the contacts 128a and 128b are shownin the state which is normal when their actuator 128 is de-energized.

Having described the manner in which the electrical output of thegenerator 74 is subject to control in response to the signal supplied atterminal 82, there will now be described the manner in which thefeedwater, fuel and air inputs to the boiler 10, as supplied by lines18, 12 and 16 respectively, are controlled so as to provide the desiredsteam pressure Pt in line 60 as well as the desired steam temperature Tswhile maintaining the desired oxygen concentration in the stack 48 andpreserving a balance between the boiler inputs and the turbine input.

The signal provided on line 116, namely Pl/Pt which is representative ofthe effective opening of valve 90, is therefore also representative ofthe energy demand established for the turbine 68 in accordance with thedesired generation from generator 74 as established by the signal Gd.The turbine energy demand signal supplied on line 116 is subject tomodification in accordance with the deviation of the throttle pressurePt measured in line 60 from its desired value or set point. Thismodication is accomplished by utilizing the output signal of thepressure measuring device 62 which is supplied on line 140 as an inputto controller 142 where it is compared with the set point for steampressure as set in the controller. The controller has proportional,integral and derivative action so as to provide an output on line 144which acts as an input to the multiplier 146 whose other input isderived from line 116. Then the output of the multiplier on line 148 isadded to the signal on line 116 by the summing unit 150 so as to providean output on line 152 which is a signal representative of the desiredboiler demand. In the usual control system the desired boiler demandsignal, DBD, on line 152 would correspond with the required boilerdemand signal, RBD, which is shown in FIG. l as appearing on line 154.That signal on line 154 would then be utilized as one of the separatedemand signals for controlling the inputs to the boiler 10 and inparticular it would be utilized to control the feedwater control to theboiler 10, thus the signal from line 54 is supplied over line 156 as afeedwater demand signal to the comparator 158 where it is compared withthe output of the owmeter 44 which is supplied on line 160 to thecomparator 158. There is then produced as an output from comparator 158on line 162 an error signal eW representing the deviation of thefeedwater flow in line 18 from the desired feedwater flow as establishedby the demand signal on line 156. The error signal eW is utilized as aninput to the controller 42 which then establishes in response to thaterror signal a change in opening of the valve 40 so as t attempt tocontrol the feedwater ow through line 18 to a value corresponding tothat represented by the signal on line 156.

In order to maintain the desired relationship between the feedwater flowand both the fuel flow and air ow, the signal RBD `on line 154 ismodified by the multiplier 166 in response to the deviation of the steamtemperature TS from its set point. The steam temperature measuringinstrument 52 supplies a signal on line 168 representing the measuredtemperature while the set point is established in the controller 170.The signal on line 168 is supplied to the controller 170 which is acontroller providing proportional, integral and derivative action inresponse to the deviation of the measured steam temperature from the setpoint to produce the output signal which appears on line 172.

The output line 172 is introduced as one input to the multiplier 166,the other input being from line 154 so that the output of the multiplieron line 174 to the summing unit 176 provides a modifying signal forestablishing on the output line 178 of the summing unit 176 anotherseparate demand signal which can be utilized to establish the requiredfuel flow to the boiler as needed to maintain the desired temperature inthe steam line 60 whenever the feedwater flow in line 18 is inaccordance with that value represented by the signal on line 156. Thesignal on line 178 is introduced by Way of line 180 to the comparator182 Where it is compared with the fuel flow signal on line 184 derivedfrom flowmeter 34. The result of the comparison provides an error signalef on line 186 which then provides an input to controller 32 by way ofline 188 so that the controller 32 can adjust the valve 3,0 to maintainthe fuel ow in line 12 at a value corresponding With the fuel demandestablished by the signal on line 180.

The desired relationship between the fuel flow and the air ow isnormally maintained by responding to deviations of the oxygenconcentration in the stack 48 of the boiler 10 from its desired value;thus there is provided a multiplier 190 which has as its other input asignal from line 192 which signal is the output of controller 194 whoseinput is derived from line 196 and whose input is thereforerepresentative of the deviation of the oxygen concentration in the stack48 from its de- `sired values as determined from the oxygenconcentration measuring instrument 50 and the set point adjustment so asto provide for the establishment of the desired oxygen concentration.The output of the multiplier 190 on line 200 is summed with the signalon line 178 j by the summing unit 202 so as to produce on the outputline 204 of the summing unit 202 a separate demand signal representativeof the demand for air flow through line 16 as required to maintain thedesired oxygen concentration in the boiler stack 48 When the fuel How inline 12 is in accordance with the demand established by the signal online 180. The signal on line 204 is compared with the signal on line 206which is derived from the flowmeter 24 and represents a measure of theair flow in line 16. The signal on line 206 is compared with the signalon line 204 by the comparator 208 so that there is provided on theoutput line 210 an error signal e,L which provdestan input to thecontroller 22 so that the controller can adjust the valve to modify theair ow in line 16 so as to maintain as closely as possible the air flowdemand as established by the signal on line 204.

In order to provide for a balance between each of the inputs to theboiler, namely the feedwater, fuel and air,

and to regulate those inputs in accordance with the variable turbineinput as established by adjustment of the throttle valve underconditions when any one of the boiler inputs may be limited as to therate at which it can change or to its magnitude, it isI necessary toprovide a means for establishing a signal RBD which is different fromthe signal DBD so that the boiler inputs will be maintained in properbalance. It is also necessary to provide means for modifying thethrottle valve 90 when any one of the `inputs to the boiler is limitedto its magnitude or rate of change so that the input to the turbine isrelated to the boiler inputs. The means for providing the signal` RBDdifferent from the signal DBD and for providing a modification of thethrottle valve 90 is shown in FIG. 1 as a block 220 which has as itsinputs the several error signals ew, ef and ea supplied from lines 162,186 and 210, respectively, as well as an input from line 152representing the signal DBD which signal would correspond with thesignal RBD1 provided as an output on block 220 whenever the respectiveerror signals ew, ef and e,L are zero or are less than a predeterminedallowable magnitude. Other outputs of the block 220 are the signalssupplied on lines 222 and 224 which, respectively, serve to actuate therelays 134 and 124 to open their respective contacts 134g and 124a.Opening the contact 134e, for example, would prevent the controller 112from causing an increase in the output of generator 74 while the openingof the contact 124a Would prevent the controller 112 from causing adecrease in the output of the generator.

Other outputs of the block 220 are the signals Obd supplied on line 228and the signal Obu supplied on line 230, which respectively energizesthe relay actuators l and 128 so as to provide for the decrease of theopening of valve 90 when the signal Obd appears on line 228 and `so asto increase the valve opening when the signal Obu appears on line 230.It will be evident that if a signal appears on line 228 to energizerelay 130, the contact 130b will be opened and the contact 130:1 will beswitched to connect to contact 130C so as to provide a path from thepulse generator 240 to line 91. On the other hand, upon the appearanceof the signal Obu on line 230 to energize relay actuator 128, thecontact 128b will be connected to the contact 128e and the contact 1280will be opened. The closing of the contact 12811 on contact 128e willallow for pulses from the pulse generator 240" to line 92 so as to causethe governor 86 to increase the opening of valve 90 and hence increasethe turbine demand. The opening of the contact 128a will prevent anypulses generated by the controller 112 from causing a signal on line 91just as the opening of the contact 130b in response to the appearance ofthe signal Obd prevented any signal from appearing on line 92 asa resultof the output of the controller 112.

The particular circuit which may be used for the unit shown as block 220and identified as the flow balance interlock unit, FBI, may be any oneof a number of circuits` of which FIG. 2 is an example.

Referring to FIG. 2 it will be evident from the following descriptionthat if the error signals on lines 162, 186 and 210 are either zero orwithin predetermined values, then the signal appearing on line 1152,namely the desired boiler demand, DBD, will cause a comparable signal toappear on line 154 as the required. boiler demand, namely RBD. Thisresults from the fact that the operational amplier 300, which is of thedifferential type, has an input resistor 302 at the inverting inputequal to the feedback resistor 304 which connects the output of theamplifier 300 to the inverting input by way of diode 308 so as to form aunity gain amplification from the line 152 to the line 310. Similarly,the operational amplifier 312 has the input resistor 314 connected totheinverting input from line 310 with a value equal to that of thefeedback resistor 316 which connects the inverting input with the outputline 318 of amplifier 312 by Way of diode 320 to form another unity gainamplifier so that there appears on line 322, and

hence on line 154, a signal of the same polarity and the same magnitudeas that appearing on line 152.

The amplifier 300 has its non-inverting input connected by way ofresistor 324 to a ground connection. Similarly, the amplifier 312 hasits non-inverting input connected by way of resistor 326 to the groundconnection.

If, for example, the signal on line 152 has a range from zero to '+8volts, the signal which will appear on line 154 will also have a similarrange. Since the signal on line 152 would be positive in polarity, thesignal appearing on line 306 would be negative in polarity and feedbackcurrent would flow through resistor 304 and line 310 and diode 308 sothat the line 310 would be negative in polarity and of approximately thesame potential as line 306, assuming no drop across the diode 308. Witha negative signal on line 310 current fiow through the resistor 316 willbe in the direction shown by the arrow, likewise current fiow throughthe diode 320 will be in the forward direction, namely that shown vbythe arrow. The source of the current through resistor 316 and thecurrent through diode 320 is the potential lE which is shown as suppliedat terminal 330 which is supplied through a small resistor 332 to line322. The amplifier 312 will serve to provide a signal at output line 318of positive potential but of such magnitude as to be less positive thanthe signal on line 322. Assuming no drop through the diode 320, thepotential at line 318 will be the same as that on line 322 and will bemaintained by the amplifier 312 such that the current in the feedbackcircuit through resistor 316 maintains the junction point betweenresistors 314 and 316 at zero potential; hence the potential at line 322will be maintained equal to that on line 310 but of opposite polarity.

Whenever one of the error signals ew, ef or ea exceeds a predeterminedvalue, it is desirable to cause the signal RBD on line 154 to representa modified value of the sig nal DBD on line 152 rather than the samevalue and as previously mentioned. If, for example, the error signal ewwhich appears on line 162 can vary from -10 to +10 volts, the networkwhich includes operational amplifier 338W can be so arranged that whenthe signal on line 1 62 deviates from zero in a negative direction or ina negative polarity by an amount in excess of a predetermined value, thediode 340W can become conductive so that the line 310 will become morenegative than the line 306 thus backbiasing the diode 308 and causingthe amplifier 338W to be the amplifier which is effective to determinethe potential on line 310.

To establish the predetermined allowable limit whlch the error signal eWmust exceed in order to cause the amplifier 338W to be effective inestablishing the potential on line 310, there is provided an adjustablecontact 342W on potentiometer 344W which is adjustable in accordancewith the magnitude of that predetermined limit. It will be noted thatone of the inputs to the non-inverting input of amplifier 338W is by wayof resistor 346W from line 152 to the summing junction 348W at thenon-inverting input. Another input to the summing junction 348W is byway of resistor 350W from line 351 which is connected to the terminal330' and hence to the potential source t-l-E. The input from the contact342W to the summing junction 348W is by way of input resistor 352W.Effectively, the potential y-l-E supplied by way of line 351 and inputresistor 350W represents the predetermined allowable limit which shouldnot be exceeded by the error signal ew, and the input signal from line152 through input resistor 346W to the summing junction 348W modifiesthe predetermined limit which the error signal ew should not exceed,which modification is made in accordance with the magnitude of therequired boiler demand signal, so that the predetermined limit, whichthe error signal ew must exceed before amplifier 338W determines thepotential on line 310, varies with the magnitude of the desired boilerdemand signal on line 152.

Since the summing junction 348W at the non-inverting input of amplifier338W is effective to compare the inputs through the three inputresistors 346W, 350W and 352W, it is possible to effectively modify thepredetermined limit which the error signal must exceed by modifying thatportion of the error signal itself which is utilized as an input to theresistor 352W, namely by the adjustment of contact 342W rather than bymaking a similar adjustment of the other inputs. Thus, since the inputson lines 152 and 351 are both positive in polarity, the signal on line162 must be negative in polarity by a certain amount to cause thesumming junction 348W to go negative so that the diode 340W will beconductive to cause the amplifier 338W to determine the potential online 310 by back-biasing diode 308. When the error signal ew causes theamplifier 338W to determine the potential on line 310, the feedbackcircuit through capacitor 354W and resistor 356W to the inverting inputof amplifier 338W from line 310 will have a current fiow in thedirection shown by the arrow causing the network including amplifier338W to act as an integrating network so that the potential on line 310is constantly increased in value with a negative polarity as long as theerror signal ew is negative by a sufficient amount to keep summingjunction 348W negative. The inverting input of amplifier 338W is alsoconnected as shown by a resistor 358W to a ground connection.

The above operation of the circuit including amplifier 338W would occurif the signals ef and ea are either zero or are within the predeterminedlimits established for them.

Each of the other error signals ef and ea, is connected to a similarnetwork as is error signal ew; thus the error signal ef is connected toa network which includes arnplifier 338)c while the error signal ea isconnected to a network which includes amplifier 338a. The other elementsof the network have reference characters comparable to those shown inthe network including amplifier 338W with the exception that the suffixletter f or "a is used depending on whether it is associated with thenetwork for the error signal ef or the network for the error signal ea.

The networks which include the amplifiers 300, 338W, 338f and 338a andtheir associated diodes 308, 340W, 340)c and 340a not only are effectiveto compare the error signals ew, ef and ea with the predetermined valuesrepresenting their allowable limits but also by virtue of the diodes308, 340W, 340]c and 340a act as an auctioneering circuit which iseffective to produce a negative potential on line 310 which is themaximum of the negative potentials produced at the outputs of theamplifiers 300, 338W, 338]c and 338:1.

If', for example, the feedwater iiow to the boiler exceeds the feedwaterdemand as established by the signal on line 156 of FIG. l, then theerror signal appearing on line 162 is negative and when that signalbecomes negative to a sufficient degree that it exceeds thepredetermined allowable limit as established by the setting 342W for theparticular signal appearing on line 152 at the time, then the amplifier338W will be effective to establish the potential on line 310 and thatpotential will be effective through the operation of amplifier 312 andits associated network to produce on line 154 a higher positivepotential than would have been produced had the error signal eW notexceeded its predetermined allowable limit. Hence, the required boilerdemand is increased if the feedwater fiow exceeds the demand previouslyestablished, as for example by the signal from line 152I by way ofamplifier 300 and amplifier 312. By increasing the required boilerdemand signal on line 154 it will be evident from FIG. l that there willbe a comparable increase in the separate demand signals produced onlines and 204 for the fuel and air flow so that an increased fuel supplyand an increased air flow will be called for to match the excessivefeedwater flow and to thereby maintain the balance between the severalinputs to the boiler, namely the feedwater, fuel and air as re- 9 quiredby the existing temperature in the output steam line 60 as well as theoxygen concentration in the stack 48.

When all of the error signals ew, ef and ea are either within thepredetermined allowed limits at the particular desired boiler demandvalue established on line 152 or when those error signals are positivein polarity, the diodes 340W, 340]'c and 340a are back-biased and theamplifier 300 and its associated network determines the potential online 310. As pointed out previously, the potential on line 310determines the potential on line 322 and hence the signal RBD by virtueof the conduction through diode 320, as long as the error signals do notexceed their predetermined allowable limits. However, if thosepredetermined allowable limits are exceeded in such a direction that oneof the error signals, as for example ew, is positive in polarity and itsvalue in excess of the value of the set limit, then the amplifier 360Wand its associated network will be effective to determine the potentialon line 322 rather than that potential being determined by the amplifier312. This is accomplished by a network which is somewhat similar to thatshown for amplifier 33'8w.

A predetermined portion of the error signal determined by the adjustedallowable limit as established by the contact 342W on the resistor 344Wprovides a positive potential on line 361W which is introduced throughthe input resistor 362W to the summing junction 364W at the invertinginput of' the operational amplifier 360W which is an amplifier of thedifferential type as are all of the other amplifiers in FIG. 2. Anotherinput to the summing junction 364w is provided from line 310 by way ofthe input resistor 366W while the third input to the summing junction364W comes from the terminal 367 which is supplied with a potential --Eand is introduced through the input resistor 368W to summing junction364W. The input connection to the non-inverting input of amplifier 360Wis from ground through resistor 365W.

The potential -E is representative of the allowable limit on the errorsignal eW while the input signal from line 310 represents the amount bywhich the allowable error signal is modified as the desired boilerdemand signal on line 152 varies, since the potential on line 310 willvary with the signal on line 152 but will be of opposite polarity. Thus,the signals provided from line 310` and from terminal 367 are negativein potential while the signal supplied on line 361W is positive inpolarity. Whenever that positive polarity input signal causes thesumming junction 364W to go positive, the amplifier 360W will begin todetermine the potential on line 322 since the diode 370W will becomeconductive and current will flow also in the integrating feedbackcircuit consisting of capacitor 372W and resistor 374W so that thepotential on the output line .376W of amplifier 360W will become lesspositive than the potential at output line 318 of amplifier 312, thusdrawing more current through diode 370W and resistor 332 so as to causethe potential on line 322 to decrease and hence provide a requiredboiler demand signal on line 154 representative of a lower requiredboiler demand and hence a lower feedwater flow demand that waspreviously required before the error signal eW exceeded its allowablelimit. With the integrating feedback circuit consisting of capacitor372W and resistor 374W, the potential on line 154 will continue todecrease as long as the error signal ew exceeds its allowable limit andhence the feedwater demand signal will constantly decrease until thedeviation of the feedwater l flowfrom the feedwater demand is at orwithin the allowable limit.

If' any of the other error signals ef or ea exceeds its predeterminedallowable limit by an amount such that the potential on thecorresponding lines 3611 or 361a exceeds the signal on line 361W, thenone of the other amplifiers 360]c or 360a will be operable to determinethe potential on line 322 since it will establish on its output line376]c or 376a a potential of lower positive value than that previouslyestablished, thus making the associated diodes 370]c or 370a conductiveand causing the other diodes connected to line 322 to be back-biased.Thus,vthe amplifiers 312, 360W, 3601 and 360a with their associateddiodes 320, 370W, 37'0f and 370a form an auctioneering circuit which iseffective to produce on line 154 a signal `for the required boilerdemand which represents the lowest boiler demand which would berequired, in other words the required boiler demand determined by theerror signal ew, ef or ea, Whichever is the greatest positive value.Since the error signals ew, ef and el are positive when the particularflow is less than the associated demand signal, when any one of thoseerror signals exceeds its predetermined allowable limit, the RBD signalis decreased in its positive value so as to become more nearly equal tothe flow of the particular quantity which has been limited. and, ofcourse, since RBD has decreased in its positive value, all of the inputsto the boiler will receive decreased demand signals and the decreasewill continue to change in magnitude so as to constantly reduce thedemand signal RBD until all of the error signals ew, ef and e, are at orwithin the predetermined allowable limits. In other words, theappropriate one of the amplifiers 338W, 338]l and 338a or 360W, 360i and360o will integrate to force the signal at their respective inputjunctions (such as 348W for amplifier 338W) to zero and thus clamp thecontroller errors to their limit whenever the error tries to exceed thelimit. As has been set forth, this action results from the loopsestablished. For example, the output of amplifier 338W or 360W connectsthrough lines 154, 156 and 162 back to the input circuit of 338W and360W.

When by virtue of one of the error signals ew, ef or ea exceeding itsallowable limit in either a positive or negative polarity, the potentialon line 310 or the potential on line 322 is established by an amplifierother than amplifier 300 and amplifier 312, respectively, there will beproduced a signal on either line 380 or 382 to the corresponding relayamplifiers 384 or 386` so as to produce a signal on either line 222 or224 which will be effective, as mentioned in the description of FIG. l,to disconnect the controller 112 'to prevent a change in the turbineinput by adjustment of valve 90. In other Words, whenever the errorsignals exceed the predetermined limit, the turbine is prevented fromhaving its input modified from the load control circuit. For example, ifthe signal on line 306 goes positive, that indicates that one of thesignals ew, ef or esu is sufliciently negative to cause the potential online 310 to be determined by an amplifier other than amplifier 300 andhence it is neces sary to prevent the controller 112 from causing adecrease to occur in the opening of the valve 90.

Likewise, when one of the error signals ew, ef or e8L is sufiicientlypositive to cause the potential on line 322 to be determined by anamplifier other than amplifier 312, then the signal from line 224 shouldprevent the controller 112 from causing an increase of the opening ofvalve 9o.

Whenever the signal DBD on line 152 exceeds the signal RBD on line 154by an amount in excess of a predetermined allowable limit as establishedby the adjustment of contact 392 on potentiometer 394, then an outputappears on line 228, `whereas if the signal DBD is less than the signalRBD by an amount in excess of that allowable limitation set by thepositioning of contact 392, then a signal will be produced on line 230.

The signal DBD is introduced to the comparator 390 by way of line 396and the signal RBD is introduced by way of line 398.

The diode 399 is connected between line 322 and ground so as to preventline 322 from going negative.

In the circuit of FIG. 1 the controllers 22, 32 and 42 `may be of thetype shown and described in U.S. Pat.

l l 2,666,170. The controllers '100, 142, 170 and 194 can be of the typedescribed in U.S. Pat. 3,092,321.

The various components of FIG. 2 may have the following values:

Component: Value 302 40K 304 40K 324 20K 316 40K 326 20K 358W, 358]c and358a 100K 356W, 356f and 356a 200K 346W, 346]c and 346a M 350W, 350f and350a 15M 352W, 352]c and 352a 100K 368W, 368f and 368a M 366W, 366)c and366a 10M 362W, 362f and 362a 100K 365W, 365f and 365a 100K 374W, 374]cand 374a 300K 354W, 354f and 354a 4/rf. 372W, 372f and 372a 4,af. 344W,344]", 344:1 and 394 2K It will be evident to those skilled in the artthat the method disclosed can be carried out by a properly programmeddigital computer. Furthermore, the logic functions shown by the relays124, 134, 122, 132, 130 and 128 and their associated contacts may alsobe carried out by other logic devices including solid state logic,lluidic logic or any other type of logic.

It Will also be evident to those familiar with boiler control that themultiplication of the signals on lines 1116 and 144 may be replaced byan addition of those signals or a subtraction depending on the desiredmode of modilication. Likewise, the multipliers 166 and 190 may beomitted if desired thus making the temperature measurement Ts and the O2measurement unnecessary in the control system shown. Still anotherapproach would be to introduce preset Values on lines 172 and 192 basedon the desired fuel-feedwater ow ratio and the air-fuel ratio,respectively.

What is claimed is:

1. In a control system which produces a signal representing the desiredboiler demand to supply the boiler load and derives from the desiredboiler demand signal separate demand signals associated with theseparate inputs to the boiler for controlling those inputs so that theytend to have a predetermined relationship and a magnitude required tomeet the boiler load, the improvement which comprises means formodifying the desired boiler demand signal so that it produces modifiedseparate demand signals in response to an error signal varying inaccordance with the difference between the actual value of a boilerinput and the corresponding separate demand signal when that errorsignal exceeds a predetermined allowable limit, said modification beingsuch that the modified demand signal and the separate demand signalsderived therefrom tend to reduce said error signal to said predeterminedlimit While maintaining the inputs in said predetermined relationship.

2. In a control system which produces from a measure of the energydemand for a turbine a signal representing the desired boiler demand tosupply the turbine demand and derives from the desired boiler demandsignal separate demand signals asociated with the separate inputs to theboiler for controlling those inputs so that they tend to have apredetermined relationship and a magnitude required to meet the turbinedemand, the improvement which comprises means for modifying the desiredboiler demand signal so that it represents the required boiler demandand produces modified separate demand signals in response to an errorsignal varying in accordance with the dierence between the actual valueof a boiler input and the corresponding separate demand signal when thaterror signal exceeds a predetermined allowable limit, said modiicationbeing such that the modified demand signal and the separate demandsignals derived therefrom tend to reduce said error signal to saidpredetermined limit While maintaining the inputs in said predeterminedrelationship.

3. A control system as set forth in claim 2 which includes meansresponsive to a predetermined difference between the desired boilerdemand signal and the required boiler demand signal and operable to varythe steam flow to the turbine so as to tend to maintain said differencebetween the desired and required boiler demand signals within thepredetermined magnitude.

4. A control system as set forth in claim 2 in which the means formodifying the desired boiler demand signal includes auctioneeringcircuits operable to modify said desired boiler demand signal to producethe required boiler demand signal so that it varies in response to thaterror signal which exceeds its predetermined allowable limit by thegreatest amount.

5. A control system as set forth in claim 4 in which the auctioneeringcircuits include a rst auctioneering circuit which modies the desiredboiler demand signal in response .to error signals 0f one polarity and asecond auctioneering circuit which modies the desired boiler demandsignal in response to error signals of an opposite polarity.

6. A control system for controlling the inputs to a boiler and to aconnected turbine so that the inputs to the boiler have a predeterminedrelationship to each other and to the turbine input comprising means formeasuring the turbine demand,

means for producing a signal indicative of said turbine demand,

means for modifying said turbine demand signal in response to thedeviation of the steam pressure in the boiler output line from itsdesired value to produce a desired boiler demand signal indicative ofthe input required to the boiler to provide the measured turbine demandand the desired steam pressure,

means for measuring the rate of supply of each of said boiler inputs,

means for producing from said rate of supply measurements separatesignals each indicative of the flow rate of one of the boiler inputs,

means for producing from said desired boiler demand signal separatedemand signals each indicative of the value of a separate one of saidboiler inputs required to meet the desired boiler demand,

means for producing an error signal for each of said boiler inputs inaccordance With the difference between the flow rate signalsrepresenting the measured value of each of said inputs and thecorresponding separate demand signals,

means for controlling said boiler inputs in direction and extent toreduce said error signals toward zero, and

means for modifying said desired boiler demand signal to produce asignal indicative of the required boiler demand said required boilerdemand signal being different from said desired boiler demand signal andproducing correspondingly modied separate demand signals when one ofsaid error signals exceeds a predetermined allowable limit.

7. The method of controlling a boiler which produces a signalrepresenting the desired boiled demand to supply the boiler load andderives from the desired boiler demand signal separate demand signalsassociated with the separate inputs to the boiler for controlling thoseinputs so that they tend to have a predetermined relationship and 13 amagnitude required to meet the boiler load, including the step ofmodifying the desired boiler demand signal so that it represents therequired boiler demand and produces modified separate demand signals inresponse to an error signal varying in accordance with the differencebetween the actual value of a boiler input and the correspondingseparate demand signal when that error signal exceeds a predeterminedallowable limit, said modification being such that the required boilerdemand signal and the separate demand signals derived therefrom tend tomaintain the inputs in said predetermined relationship and to maintainsaid error signal below said predetermined allowable limit.

8. The method for controlling the inputs to a boiler and to a connectedturbine so that the inputs to the boiler have a predeterminedrelationship to each other and to the turbine input comprising the stepsof measuring the turbine demand,

producing a signal indicative of said turbine demand,

modifying said turbine demand signal in response to the deviation of thesteam pressure in the boiler output line from its desired value toproduce a desired boiler demand signal indicative of the input requiredto the boiler to provide the measured turbine demand and the desiredsteam pressure,

measuring the rate of supply of each of said boiler inputs,

producing from said rate of supply measurements separate signals eachindicative of the How rate of one of the boiler inputs,

producing from said desired boiler demand signal separate demand signalseach indicative of the value of a separate one of said boiler inputsrequired to meet the desired boiler demand,

producing an error signal for each of said boiler inputs in accordancewith the difference between the ow rate signals representing themeasured value of each of said inputs and the corresponding separatedemand signals,

controlling said boiler inputs in direction and extent to reduce saiderror signals toward zero, and

modifying said desired boiler demand signal to produce a signalindicative of the required boiler demand said required boiler demandsignal being different from said desired boiler demand signal andproducing correspondingly modified separate demand signals when one ofsaid error signals exceeds a predetermined allowable limit until saidone error signal does not exceed its allowable limit.

9. The method of claim 8 which includes the steps of modifying the steaminput to the turbine from the boiler when said required boiler demandsignal differs from said desired boiler demand signal by an amount inexcess of a q predetermined allowable limit, said modification being indirection and magnitude to maintain the diierence between the desiredboiler demand signal and required boiler demand signal within saidallowable limit.

10. The method of claim 8 which includes the steps of producing theseparate demand signal associated with the feedwater input to the boilerdirectly in accordance with said required boiler demand signal,producing the separate demand signal associated with the `fuel ow inputto the boiler by modifying the required boiler demand signal in responseto the deviation of the steam temperature in the boiler output line fromits desired value, and producing the separate demand signal associatedwith the air flow input to the boiler by modifying the fuel flow demandsignal in response to the deviation of the oxygen concentration in theboiler stack from its desired value.

References Cited UNITED STATES PATENTS 3,388,553 6/1968 Anderson 60-106X3,417,737 12/ 1968 lShinskey et al. 122-448 MARTIN P. SCHWADRON, PrimaryExaminer A. M. OSTRAGER, Assistant Examiner U.S. Cl. X..R. 122-448

